Most rhythmic motor patterns such as swimming, walking, chewing, and breathing are coordinated by neural networks called central pattern generators (CPGs). The ability of these networks to produce rhythmic output is determined by the membrane currents of the cells and the synaptic connections among the cells. Studies of a variety of vertebrate and invertebrate systems have shown that endogenous peptide modulators alter the output rhythm by modifying synaptic connections and/or membrane currents of neurons in the network (Dale and Kuenzi, 1997). Characterizing how modulators work aids our understanding of how rhythmic networks function and how they can respond adaptively to the environment by neuromodulation. This knowledge will advance understanding of CPG dysfunction, such as occurs in some respiratory and ambulatory disorders (Feldman and Gray, 2000; Feraboli-Lohnherr et al., 1999). Because of its small size and well-defined anatomy, the 14-interneuron network, controlling the leech heartbeat is a model system for exploring rhythmic network function. Within this network, two pairs of oscillator interneurons create the underlying alternating rhythm of the leech heartbeat. This proposal focuses on understanding how a neuromodulator acts on these oscillator interneurons to accelerate the heartbeat pattern. In addition, this proposal discusses the development of a computer model of these interneurons incorporating information about cell morphology to understand how morphology and ion channel distribution contribute to rhythmic activity. This series of experimental and modeling studies clarifies how neural networks produce rhythmic activity and how neuromodulation of current kinetics affects the rhythmic output.